Multiple objects following the same path at different rates
With just a follow path constraint, with fixed position set to True, an offset of 0 is start, 1 is end of curve. The offset can be keyframed / driven.



Sample file, 3 cubes, All at 0 offset at frame 1.  Two are keyframed, one traverses path in 50 frames the other 80 with an offset value of 1 at those frames respectively.

The other is driven by the frame number

(frame - 1) / 100


which will be at 0 at frame 1 and 1 offset at frame 101 and above.